In general, a color cathode-ray tube is provided with an envelope that includes a rectangular face panel having side wall sections on a peripheral edge portion of an effective section thereof, and a funnel coupled to the side wall sections of the panel. A phosphor screen comprising three color phosphor layers which are able to emit blue, green and red light is formed on the inner surface of the effective section of the face panel. In the envelope, a substantially rectangular shadow mask is opposed to the inside of the face panel. An electron gun for emitting three electron beams is disposed in the neck of the funnel.
The electron beams emitted from the electron gun are deflected by a deflecting device mounted on the outer surface of the funnel, and are used to horizontally and vertically scan the phosphor screen through the shadow mask, thereby displaying a color image.
The shadow mask serves to sort out the three electron beams from the electron gun and then correctly land them onto the three color phosphor layers, in order to obtain desired colors. The shadow mask has a substantially rectangular shadow mask body with multiple electron beam passage apertures, and a substantially rectangular mask frame attached to the periphery of the shadow mask body. At least three side walls of the mask frame are supported on the face panel side walls by means of elastic holders. Each holder has an end portion fixed to the mask frame, and the other end portion engaged with a stud pin which is provided on the inner surface of a corresponding side wall of the face panel.
In color cathode-ray tubes having a shadow mask, only 30% or less of the electron beams emitted from the electron gun pass through the electron beam passage apertures of the shadow mask body and reach the phosphor screen, whereas about 70% of the electron beams strike upon the shadow mask body. As a result of the striking of the electron beams, the shadow mask is heated and thermally expanded. When, in particular, displaying an image of a high luminance, the relative positions of the electron beam passage apertures to the phosphor screen change due to the thermal expansion of the shadow mask body and the mask frame, thereby disabling the electron beams, having their spots shaped by the shadow mask body, to strike upon or land on respective phosphor layers. As a result, color purity degradation will occur.
The cause of such color purity degradation during the operation of the color cathode-ray tube is mainly classified into two types, i.e. thermal expansion of the shadow mask body and that of the mask frame.
Color purity degradation due to the thermal expansion of the shadow mask body occurs at the initial time of high luminance image display, and the landing position of each electron beam is displaced from a predetermined position in the radial direction toward the center of the phosphor screen. This displacement is caused by a doming phenomenon, which occurs when the shadow mask body of a small heat-capacity is mainly heated while the mask frame of a large heat-capacity is not greatly heated, and in which phenomenon the shadow mask body expands toward the phosphor screen.
The doming is a phenomenon in which the outward size of the mask frame does not change and the shadow mask body thermally expands and swells upon the phosphor screen, and causes the landing positions of the electron beams to be displaced toward the center of the screen (hereinafter, color purity degradation due to the doming phenomenon will be referred to as a "PD-1").
The PD-1 can be suppressed by making the shadow mask body of a material of a low thermal expansion property to thereby reduce the degree of mask doming due to the thermal expansion of the shadow mask.
Further, the color purity degradation due to thermal expansion of the mask frame (hereinafter referred to a "PD-2") occurs when the landing position of each electron beam is displaced from a predetermined position toward the radially outside of the phosphor screen. This displacement is caused when the outward size of the mask frame increases to thereby relax the doming phenomenon, while a peripheral portion of the shadow mask body is pulled by the mask frame, as a result of heat transmission from the shadow mask body to the mask frame.
A method for correcting color purity degradation due to the thermal expansion of the mask frame is proposed, in which elastic members for supporting the mask frame on the face panel, i.e., holders, have their shape, material, etc. modified appropriately.
Specifically, each of the holders, for example, is formed by bending an elongated metal plate, and comprises a fixed portion fixed to the mask frame, an engagement portion having an engagement hole to be engaged with a corresponding stud pin projecting from the face panel, and a slope portion extending between the fixed portion and the engagement portion. The metal plate is bent along a first bending line located between the fixed portion and the slope portion, and also along a second bending line located between the slope portion and the engagement portion. The first and second bending lines extend at an angle to the direction which is perpendicular to the longitudinal axis of the holder.
Where the mask frame is supported by such holders, when the mask frame thermally expands and compresses the holders, they are elastically deformed in a direction in which their bent portions extend, thereby displacing the mask frame toward the phosphor screen. In accordance with this displacement, the shadow mask body as well moves toward the phosphor screen. As a result, the landing positions of the electron beams are corrected and the color purity degradation is suppressed.
The amount of displacement of the mask frame along the axis of the tube, which determines the correction amount for the PD-2, is determined on the basis of the height of each bent holder, and the angle of the first and second bending lines. To increase the correction amount for the PD-2, it is necessary to reduce the angle of the bending lines with respect to the longitudinal central line of each holder, and to increase the displacement amount of the mask frame along the tube axis.
The displacement amount of the mask frame due to thermal expansion is determined depending upon how firmly the holders are fixed on the mask frame. In other words, to obtain a predetermined displacement amount of the mask frame, it is important to reliably secure the holders to the mask frame.
However, in the above-described holder, the fixed portion has the same width as the slope portion, and therefore the acute-angle portion of the fixed portion, which is defined by side edges of the fixed portion and the first bending line, contacts the mask frame by only a small area. This makes it difficult to secure, in the fixed portion, a sufficient portion necessary for spot welding. As a result, the fixed portion is welded at a portion away from the acute-angle portion, which means that the acute-angle portion of the fixed portion, which is the root of the slop portion, cannot be firmly secured to the mask frame. Similarly, depending upon the shape of the mask frame, the fixed portion of the holder cannot firmly be secured thereto.
Accordingly, the acute-angle portion will separate from the mask frame during elastic deformation of the holder, thereby causing a gap to be defined between the fixed portion and the mask frame. The result is that the displacement of the mask frame along the tube axis is deviated from a desired set value while the mask frame thermally expands, and therefore the PD-2 cannot be corrected satisfactory.